Are you searching for the best online ER diagram tool? Read on and discover our top picks. Data modeling is one of the most critical parts of a software project. Database architects use ER diagrams (also called ERDs or Entity Relationship Diagrams) as a data modeling technique. Many ER diagram tools are available, but choosing the best tool for your project can be hard. Still, your data model is a key factor in the success of your entire project; it’s important to get it right.